The invention concerns an internal-combustion engine. More particularly, the invention concerns a two-cycle, opposed piston engine.
The opposed piston engine was invented by Hugo Junkers around the end of the nineteenth century. Junkers' engine uses two pistons disposed crown-to-crown in a common cylinder having inlet and exhaust ports near bottom-dead-center of each piston, with the pistons serving as the valves for the ports. The engine has two crankshafts, one disposed at each end of the cylinder. The crankshafts, which rotate in the same direction, are linked by connecting rods to respective pistons. Wristpins within the pistons link the rods to the pistons. The crankshafts are geared together to control phasing of the ports and to provide engine output. In a typical Junkers engine, a supercharger is driven from the intake crankshaft, and its associated compressor is used to scavenge the cylinders and leave a fresh charge of air each revolution of the engine. Optionally, a turbo-supercharger may also be used. The advantages of Junkers' opposed piston engine over traditional two-cycle and four-cycle engines include superior scavenging, reduced parts count and increased reliability, high thermal efficiency, and high power density. In 1936, the Junkers Jumo airplane engines, the most successful diesel engines to that date, were able to achieve a power density that has not been matched by any diesel engine since. According to C. F. Taylor (The Internal-Combustion Engine in Theory and Practice: Volume II, revised edition; MIT Press, Cambridge, Mass., 1985): “The now obsolete Junkers aircraft Diesel engine still holds the record for specific output of Diesel engines in actual service (Volume I, FIG. 13-11).”
Nevertheless, Junkers' basic design contains a number of deficiencies. The engine is tall and requires a long gear train to couple the outputs of the two crankshafts to an output drive. Each piston is connected to a crankshaft by a rod that extends from the piston. The connecting rods are massive to accommodate the high compressive forces between the pistons and crankshafts. These compressive forces, coupled with oscillatory motion of the wristpins and piston heating, cause early failure of the wristpins. The compressive force exerted on each piston by its connecting rod at an angle to the axis of the piston produces a radially-directed force (a side force) between the piston and cylinder bore. The friction generated by this side force is mitigated by a lubricant film between the cylinder and piston, but the film ruptures beyond a certain temperature and side force. Since the temperature of the cylinder/piston interface is principally determined by the heat of combustion, the breakdown temperature of the lubricant imposes a limit on the engine combustion temperature, which, in turn, limits the brake mean effective pressure (BMEP, an indicator of engine power) achievable by the engine. One crankshaft is connected only to exhaust-side pistons, and the other only to inlet-side pistons. In the Jumo engine the exhaust side pistons account for up to 70% of the torque, and the exhaust side crankshaft bears the heavier torque burden. The combination of the torque imbalance, the wide separation of the crankshafts, and the length of the gear train produces torsional resonance effects (vibration) in the gear train. A massive engine block is required to constrain the highly repulsive forces exerted by the pistons on the crankshafts during combustion, which literally try to blow the engine apart.
In an opposed piston engine described in Bird's U.K. Patent 558,115, counter-rotating crankshafts are located beside the cylinders such that their axes of rotation lie in a plane that intersects the cylinders and is normal to the axes of the cylinder bores. The side-mounted crankshafts are closer together than in the Jumo engines, thereby reducing the height of Bird's engine as compared with that of the Jumo engines. Bird's crankshafts are coupled by a shorter gear train that requires four gears, compared with five for the Jumo engine. The pistons and crankshafts in Bird's engine are connected by rods that extend from each piston along the sides of the cylinders, at acute angles to the sides of the cylinders, to each of the crankshafts. In this arrangement, the rods are mainly under tensile force, which removes the repulsive forces on the crankshafts and yields a substantial weight reduction because a less massive rod structure is required for a rod loaded with a mainly tensile force than for a rod under a mainly compressive load of the same magnitude. Bird's proposed engine has torsional balance brought by connecting each piston to both crankshafts. This torsional balance, the proximity of the crankshafts, and the reduced length of the gear train produce good torsional stability. To balance dynamic engine forces, each piston is connected by one set of rods to one crankshaft and by another set of rods to the other crankshaft. Piston load balancing substantially reduces the side forces that operate between the pistons and the internal bores of the cylinders. However, even with these improvements, traditional engine construction and conventional cooling prevent Bird's proposed engine from reaching its full potential for simplification and power-to-weight ratio (“PWR”, which is measured in horsepower per pound, hp/lb).
Bird's engine uses an engine block in which cylinders, cylinder intake and exhaust manifolds, cylinder cooling jackets and engine bearings are cast in a large, heavy unit serving as the primary structural element of the engine. Thermal and mechanical stresses transmitted through the engine block and uneven heating during engine operation cause non-uniform cylindrical distortion of the cylinders. The piston crowns bear extremely high temperatures during combustion and become distent radially as a result. The cooling system of Bird's engine provides liquid coolant through the cylinder jackets in the engine block, but the system is not adapted to mitigate the non-uniform distortion of the cylinders or to prevent expansion of the piston crowns. As a consequence, close tolerances cannot be maintained between cylinders and pistons without a high risk of engine damage or early engine failure. Of course, without close tolerances, it is difficult to provide an effective seal between cylinders and pistons to limit blowby (the escape of gasses past the piston) during engine operation, without the use of piston rings. A rigid piston structure in which connecting rods are coupled with wrist pins mounted to piston skirts over-constrains the pistons during operation of the engine. This over-constraint prevents any part of a piston from repositioning with respect to the axis of an associated cylinder in response to an imbalance of forces coupled to the piston through the connecting rods.
A two-stroke, opposed-piston engine with side-mounted, counter-rotating crankshafts is described in PCT Patent Application PCT/US2005/020553. In this engine, the working elements (cylinders, pistons, linkages, crankshafts, etc.) are received upon a frame of passive structural elements fitted together to support the working elements. The frame bears the stresses and forces of engine operation, including compressive forces between the crankshafts. In contrast with the Junkers and Bird engines, the cylinders are not cast in an engine block, nor are they formed with other passive structural elements. Consequently, the cylinders are not passive structural elements of the engine. Thus, with the exception of combustion chamber forces, the cylinders are decoupled from the mechanical and thermal stresses of an engine block and are essentially only pressure vessels. Tailored application of liquid coolant to each cylinder of the engine compensates for asymmetrical heating of the cylinders, while the symmetrical application of liquid coolant to the interior surface of each piston crown maintains the shape of piston crowns during engine operation. A single intermediate gear between the two crankshafts shortens the gear train and substantially reduces torsional resonances between the crankshafts, as compared with Bird's engine.
The engine described in PCT Patent Application PCT/US2005/020553 also includes a compliant member that allows for angular adjustment of piston structure with respect to the cylinder in response to an imbalance in forces coupled to the piston by the connecting rods. In this regard, an axially-centered tubular rod is mounted in the piston, and the connecting rods are linked to wrist pins attached to the rod. Piston compliance is realized in the innate flexibility of the tubular rod. Elimination of wrist pins from skirt mountings permits reduction of skirt mass and piston weight.
Further benefits to the engine described in PCT Patent Application PCT/US2005/020553 have resulted from additional embodiments of a compliant piston structure including a compliance boot acting between the piston crown and an axially-centered rod mounted in the piston. A single wristpin mounted on an axially-centered piston rod, externally to the piston, couples the piston with associated connecting rods that run between the piston rod and the crankshafts of the engine.